Strange situation, any ideas?

[Mysticplayer]

Mostly, I would say changes in wind. Spending alot of time practising for F class shooting, you can really see the affect wind, light/mirage and THERMALS have on POI. I have had groups move 2MOA at 300m just on condition change.

The 308 gets moved around alot so doesn't take much of a change for a big change in POI at LR. A let up in head wind WILL affect POI.

I would suggest a repeat of the shooting with the 'new' reload and see what you get. Calm conditions with overcast skies, early in the morning would be ideal.

You can get thermals anytime of year and with the terrain you shoot over, I would expect them to be quite common and very large. A small bubble can be 100yds across (some only a few ft) and last for a seconds to minute or two. Some bubbles can be 1/2 mile across.

If there is a glider club or private airfield in your area, talk to them about the weather patterns/ground effects. The upward velocity of these thermals can be very impressive. Think moving a Boeing 747 a few ft or more (called air turbulence) while it is traveling 450mph. Or making commercial planes crash on take off/landing (commonly refered to as wind shear).

Unfortunately, we have no way to watch for this occurance (no vertical wind flags, at least not yet).

Something to add to the mix of confusion.

Jerry

[goodgrouper]

More info needed to get the answer.

Was the new brass same brand/lot as old brass?
Were the bullets fired also from the same lot for both new and fireformed brass?
Did the new brass have the same neck tension as the old?
Is the chamber on the rifle sloppy and thereby allowing the fireformed cases to measure larger capacity?

[Ian M]

Grouper,
Good questions, cannot answer all tho.
Brass was different lots for sure, believe Win.
Bullets different lots also.
Previously shot brass was full sized, nothing on new brass, just inserted a bullet.
No idea about chamber measurement but expect this had something to do with it, pressures. But weird that the zeros came back into order at 6 & 7.
Might have been some thermals but this was late in the day and wind was dying out.
Got better things to worry about. New rifle. No time to shoot it. Not fair.

[Mysticplayer]

Thermals are most 'active' when there are large swings in ground vs air temp. An approaching cool air mass over a hot field will cause bubbles. As would the sun poking out from behind clouds quickly heating the ground.

Also, rapid changes in wind speed or direction ARE indicators of thermals lifting off or downdrafts filling in the air that just left.

Days with lightish winds will have these conditions more pronounced then a day with steady winds from a certain direction.

Oh by the way, thermals will move across large spans of terrain/prairie 'sucking' up air in their path. Think very mild tornado with a really fat butt/base. The backside of this is equally strong downdrafts.

Grab a book on R/C glider flying and they will give you lots of info and tips about thermals.

Jerry